RussianMultiplication

package tools

import "strconv"

/*  Russian Multiplication represeting in demical value calculate by
    Create one variable to keep carry out
    First value divide by 2 while..
    Second value multiply by 2
    If first is odd *Except one* : carry out += before-multiply second value
    Example:    6 * 6 : no carry out
            3 * 12 : carry out + 12
            1 * 24 : no carry out
    Return 24 + 12 = 36
*/
func DecimalMul(first, second int) int {
    // Create variable to keep carry out
    var result int = 0
    // First always reduce to one
    for first != 1 {
        if first%2 != 0 {
            // Get carry out
            result += second
        }
        // Divide and Multiply
        first /= 2
        second *= 2
    }
    // Current Second value + Carry out
    return second + result
}

/* Russian Multiplication represeting in binary value calculate by
    *Input : 2 Strings
    Create one variable to keep carry out
    First value delete last elemnt out while..
    Second value add one 0 in
    If last element of first value is 1 *Except when length is 1* : carry out += before-multiply second value
    *In my case: carry out of mine, I turn it to decimal, plus them, and turn it back to binary
    Example :   110 * 110 : no carry out
            11 * 1100 : carry out = 1100
            1 * 11000 : no carry out
    Return 11000 + 1100 = 100100
*/
func BinaryMul(first, second string) string {
    // Create variable to keep carry out
    var result string = "0"
    // When length of first = 1 means first = "1"
    for len(first) > 1 {
        // Check if last element of first is 1
        if first[len(first)-1:] == "1" {
            // Add carry out by sumBinary Method
            result = sumBinary(result, second)
        }
        // Delete last element out
        first = first[:len(first)-1]
        // Add 0 in
        second += "0"
    }
    // Add carry out and return
    return sumBinary(result, second)
}

/*  Add carry out to current second value
    Turn first and second value to decimal
    Plus them
    Turn them back to binary and return
    Example :   "11" + "11"
            Turn to decimal
            3 + 3 = 6
            Turn back to binary
            110
    Return "110"
*/
func sumBinary(first, second string) string {
    // ParseInt(string, Current bit, Format of int) -> value in decimal, err
    f, _ := strconv.ParseInt(first, 2, 64)
    s, _ := strconv.ParseInt(second, 2, 64)
    // FormatInt(value, base) -> string
    return strconv.FormatInt(f+s, 2)
}